Salar de Uyuni in south-west Bolivia was once part of a massive prehistoric lake but today is the world’s largest salt flat. When dry it’s a barren landscape, so featureless that it’s great for perspective tricks – but during the wet season it becomes a spectacular giant mirror. Salar de Uyuni is estimated to contain as much as 10 billion tonnes of salt, and about 25,000 tonnes is collected here each year, using the traditional method of sweeping the salt up into neat piles to dry before being carted away. However, salt is not Salar de Uyuni’s only…